Fresnel Zones http en wikipedia orgwikiFresnelzone Fresnel Zones

Fresnel Zones http: //en. wikipedia. org/wiki/Fresnel_zone

Fresnel Zones • Contents 1 Fresnel zones 2 Determining Fresnel zone clearance 3 See also 4 References 5 External links

Fresnel Zones • If unobstructed, radio waves will travel in a straight line from the transmitter to the receiver. • But if there are obstacles near the path, the radio waves reflecting off those objects may arrive out of phase with the signals that travel directly and reduce the power of the received signal.

Fresnel Zones • On the other hand, the reflection can enhance the power of the received signal if the reflection and the direct signals arrive in phase.

Fresnel Zones • Fresnel provided a means to calculate where the zones are, where a given obstacle will cause mostly in phase or mostly out of phase reflections between the transmitter and the receiver. • Obstacles in the first Fresnel zone will create signals with a path-length phase shift of 0 to 180 degrees, in the second zone they will be 180 to 360 degrees out of phase, and so on. • Even numbered zones have the maximum phase cancelling effect and odd numbered zones may actually add to the signal power. [2]

Determining Fresnel zone clearance • The concept of Fresnel zone clearance may be used to analyze interference by obstacles near the path of a radio beam. • The first zone must be kept largely free from obstructions to avoid interfering with the radio reception. • However, some obstruction of the Fresnel zones can often be tolerated, as a rule of thumb the maximum obstruction allowable is 40%, but the recommended obstruction is 20% or less.

Determining Fresnel zone clearance • For establishing Fresnel zones, first determine the RF Line of Sight (RF Lo. S), which in simple terms is a straight line between the transmitting and receiving antennas. • Now the zone surrounding the RF Lo. S is said to be the Fresnel zone. [3]

Determining Fresnel zone clearance

Determining Fresnel zone clearance •

Determining Fresnel zone clearance • The cross sectional radius of each Fresnel zone is the longest in the center of the RF Lo. S, shrinking to a point at the antenna on each end. • For practical applications, it is often useful to know the maximum radius of the first Fresnel zone.

Determining Fresnel zone clearance •